The decision to plug and abandon (P&A) a well or field is an economic decision. Once production value drops below operating expenses, it is time to consider abandonment, even if considerable reserves remain. Thus, well abandonment is an inevitable stage in the lifespan of a well.
It is also possible to abandon only part of a well. One cost effective way to enhance production is to permanently abandon the bottom of the well, but use the existing slot to sidetrack the well to reach new pay-zones. The cost can often be cut in half when sidetracking an existing well instead of drilling a new horizontal well. This process is known as “slot recovery.”
Plugging can also be temporary, e.g., to allow for work-over, a long shut-in, or for converting an exploratory well to a production well. Norwegian standards state that the integrity of materials used for temporary abandonment should be ensured for the planned abandonment period times two.
In oilfield jargon “plug and abandon” or “P&A” refers to preparing a well to be closed permanently (or at least until prices or technology developments warrant reentry). The earliest oil wells were abandoned without any plugging, but the first plugging requirements were enacted by Pennsylvania in the 1890s. However, prior to modern regulations set in the '50s, many wells were abandoned with plugs consisting of brush, wood, paper sacks, linen or any other material that could be pushed into a well to form a basis for the dumping of one or two sacks of cement to “plug” the well. Current procedures are significantly more disciplined however.
Plugging and abandonment (P&A) regulations vary among states and between countries, but all regulations prescribe the depth intervals that must be cemented, as well as the materials that are allowed in plugging practices. Most states require that cement plugs be placed and tested across any open hydrocarbon-bearing formations, across all casing shoes, across freshwater aquifers, and perhaps several other areas near the surface, including the top 20 to 50 ft [6 to 15 m] of the wellbore. Many countries and states require that a “rock-to-rock” cement plug be set that is contact with wellbore outside the casing if the casing is not adequately isolated with cement.
In recognition of its strength, low permeability and low cost, cement typically is used to create a seal between formations or to seal off the surface of the wellbore. Other materials that do not offer the same strength or durability as cement, including drilling mud, gel, and clay, are used to fill in the spaces between cement plugs. Additionally, many states allow the use of mechanical bridge plugs in lieu of a large cement plug since the bridge plug is extremely strong and nearly completely impermeable. However, mechanical plugs are susceptible to corrosion, and therefore the regulations typically require the bridge plugs to be capped by a specified amount of cement.
The basics of P&A operations vary little, whether the well is on land or offshore. Operators remove the completion hardware, set plugs and squeeze cement into the annuli at specified depths across producing and water-bearing zones to act as permanent barriers to pressure from above and below, in addition to protecting the formation against which the cement is set. Operators remove the wellhead last. Some basic plugs are shown in FIG. 1.
Balanced plug technique is the most common placement method used in abandonment operations today. A tubing or a drill string is lowered to the desired depth for the plug base and the cement slurry is pumped until the cement slurry level is the same inside and outside of the string. When the cement height is the same on the inside of the tubing as in the annulus, the pipe is slowly pulled out. The pipe will be pulled out with a speed so that the fluid level is balanced at all times. When the pipe reaches the cement-spacer interface, little or no mixing between the spacer and the cement will occur if the interfaces between the fluids are the same both inside and outside the pipe.
One of the main problems in any cementing procedure is contamination of the cement. Poor mud-removal in the area where the cement is to be set can give rise to channels through the plug caused by the drilling fluid. To avoid this, a spacer is often pumped before and after the cement slurry to wash the hole and to segregate the drilling fluid and the cement from each other.
Another cause for channeling is eccentricity of the tubing, indicating the importance of adequate use of centralizers, which hold the tubing in the center of the bore. The cement will have more difficultly moving on the narrow side of the tubing, tending to allow channeling in the narrow space, and even where channeling does not occur, the cement will be thinner on that side, and thus be weaker and more easily damaged. Cement shrinkage can also cause gaps between the plug and casing and between the plug and reservoir wall. (FIG. 2). Cement plug integrity is also influenced by the cement density, the condition of the pipe, and the additives used in the cement. The quality of the cement mixing equipment on location also plays a very large role in plug success.
Because cement is susceptible to channeling, shrinkage and other problems, most regulations require that a substantial length of well be filled with cement, ranging from 30 to 50 meters. Thus, the response to cements shortcomings is to simply use more cement, in the hopes that eventually a reliable barrier will be formed. However, a 50-meter length of cement plug can require 2 tons of cement, which is expensive and time consuming to deploy, and takes a long time to cure. Area preparation and tubular removal, which might require milling of casing strings, is also very time consuming. Where every day on an offshore rig costs as much as a half to a million dollars a day, there is a strong drive to reduce time and costs.
Other materials have been investigated for use as plugging material. Resins offer superior adhesion, resistance to many caustic and corrosive chemicals, excellent mechanical properties such as low yield point and low viscosity in the unset state, and flexibility and toughness after setting, but historically they have been difficult to deploy without premature setting and or reactivity with downhole fluids. Additionally, if resin is placed in same volume as cement, it would make resin use very expensive, probably prohibitively costly.
Today's resin materials have improved however, and include ThermaSet by Wellcem AS, CannSeal by AGR, and the WellLock® resin system by Halliburton®. M&D Industries also makes resin plugging materials, including LIQUID BRIDGE PLUG®S with a range of hardeners and accelerators. The WellLock® resin, for example, uses cross-linking between an amine hardener and epoxides, resulting in a cured three-dimensional infinite polymer network, and can be deployed without negative impact from exothermic reactions triggered by water.
New types of cement slurries consisting of geopolymeric materials have also been developed as alternative to the conventional lightweight cement slurry. Geopolymers are made of aluminum and silicon and they exhibit superior mechanical and chemical properties compared to the Class G cement. Geopolymers can provide a material with specific properties from a range of cement/fly-ash/aluminiosilicate component ratios. This gives a lightweight slurry with high compressive and flexural strength thought to replace the conventional lightweight cements containing silica fume.
Sandaband is another cement alternative. It is a sand-slurry consisting of about three quarters sand particles and one quarter water and other additives, developed in Norway to meet the increasing demands of an long lasting plugging material. Sandaband possesses the properties as a Bingham fluid and acts as a deformable solid when it's stationary, but as a liquid when in motion. This ductile behavior means that the sand slurry will never fracture or create micro annuli. The sand slurry is also incompressible and gas tight, and does not shrink, fracture or segregate. It does however require a solid foundation, as it will sink if placed on another fluid.
Today, regulators are increasingly demanding that operators remove sections of casing so that a plug may be set that is continuous across the entire borehole in a configuration often referred to as “rock-to-rock,” and located in the cap rock above the reservoir. Because cement or other plugging material must go all the way to the formation wall, the typical procedure was to pull the tubing, mill the casing, and remove swarf before spotting the cement. See FIG. 3. However, this process may require multiple trips downhole and the tons of swarf that must be removed can accumulate in low flow zones, and has razor sharp edges, being hazardous to both drill crew and equipment. Plus, the method is expensive and time-consuming.
One response to these challenges has been the introduction of a system known as perforate, wash and cement or “PWC” in a single run. The PWC operation is designed to access the formation through perforations in the casing to place a rock-to-rock cement barrier without removing the casing, thus saving valuable rig time and eliminating the swarf problem. To use this system, the well must be secured, Christmas tree removed, tubing pulled, and then PWC job can be done.
The PWC method uses a special tool by Archer, described in US20150053405. The tool is made of pipe conveyed perforating guns attached below a wash tool, which is below a cement stinger. Using PWC, ConocoPhillips completed 20 PWC plug installations in the North Sea, reducing the time required to set a permanent plug to 2.6 days from 10.5 days using section milling. As a result, the company calculated a savings of 124 rig days over the course of the 20 PWC wells. Given that rig time can easily be upwards of half a million dollars per day for an offshore rig, even a few days less time required for P&A can mean significant cost savings.
Although an improvement, the PWC method has limitations. To date, the PWC method has not been successfully applied through multiple casings. Furthermore, it is difficult to implement this method if the pipe has deformed such that the lengthy tool can no longer enter through the deviated section.
Thus, what is needed in the art are better methods, devices and systems for P&A that are safe, create a reliable barrier, that are cost effective, and both faster and easier to perform than current methods. Ideally, the new method would not require rig time, and would be performed “through tubing,” and could provide a “rock-to-rock” plug. An ideal system would not require securing the well, allow the Christmas tree to remain in place during operations acting as a barrier, avoid expensive modular offshore drilling unit or “MODU” use, and also avoid the rigging up of large BOP' s and well control equipment.